Nonfunctional, monofunctional and difunctional-terminated liquid homopolymers derived from butadiene and isoprene monomers are suitable for a variety of applications in the polymer industry, and are particularly desirable due to their relatively low cost. However, the presence of unsaturation in the aliphatic hydrocarbon backbones of these monomers render the polymers produced therefrom thermooxidatively unstable. That is, such polymers are susceptible to breakdown through oxidation catalyzed by heat. However, by hydrogenating the unsaturation present in these polymers to a high degree, the thermooxidative stability of the polymers can be substantially improved. More particularly, removal of at least about 95 percent of the unsaturation in nonfunctional, monofunctional and difunctional-terminated polybutadiene or polyisoprene significantly improves their thermooxidative stability, and saturation of at least about 99 percent of the backbone double bonds results in a polymer having superior thermooxidative stability.
Catalytic hydrogenation is a conventional way of obtaining saturated materials from nonfunctional, monofunctional and difunctional-terminated conjugated diene polymers. However, cost-effective heterogeneous catalysts, such as Raney nickel, which are suitable for the hydrogenation of most monomers are, in general, unsuitable for hydrogenation of nonfunctional, monofunctional and difunctional-terminated conjugated diene polymers to a high degree, presumably due to the inaccessibility of the polymer backbone to the catalyst. This inaccessibility is due to the coiled conformation that these polymers assume. In some instances, specially prepared heterogeneous noble metal catalysts with a high surface area, such as rhodium and palladium-based heterogeneous catalysts, have been shown to be successful in such an application, but are not cost-effective due to their inherent high cost as well as increased expense related to the special preparation of these catalysts. As alluded to above, in order for a hydrogenation catalyst to be effective, intimate contact between the catalyst and the polymer backbone is necessary. Hence, preferred catalysts for the hydrogenation of these polymers are homogeneous rather than heterogeneous, since such homogeneous catalysts achieve this intimate contact.
Rhodium and palladium-based homogeneous catalysts, and in particular tristriphenylphosphinerhodium (I) chloride, have heretofore been the catalysts of choice for hydrogenating functional-terminated conjugated diene polymers. These catalysts are chosen because they achieve intimate contact with the polymer backbone and further because they are unaffected by the functional groups of the polymers, resulting in a high degree of hydrogenation of the unsaturated polymer backbone. However, rhodium and palladium-based homogeneous catalysts are very expensive. Although more conventional homogeneous catalysts based on titanium, iron, cobalt, or nickel are generally about 1,000 times less expensive than the rhodium or palladium-based homogeneous catalysts, these conventional homogeneous catalysts are easily deactivated or "poisoned" in the presence of certain functional groups in the polymer being hydrogenated. For example, a cobalt-based homogeneous catalyst would undergo oxidative addition with the terminal hydroxy groups of a monofunctional or difunctional hydroxy-terminated polybutadiene polymer, thereby causing chain extension and subsequent gelation of a solution of the polymer, thus making the polymer unsuitable for hydrogenation. Aluminum trialkyls, which are commonly used reductants in the formation of active cobalt species, would also exert a similar effect on the polymer.
Japanese Patent 62151404 relates to hydroxy-containing diene polymers which are catalytically hydrogenated in solvents, then treated with alcohols to separate highly-hydrogenated polymers from low or non-hydrogenated polymers. Thus, 300 g of OH-terminated polyisoprene was hydrogenated over Ni-diatomaceous earth in hexane at 130.degree. and 50 kg/cm.sup.2 -gage H for 4 hours, then shaken with 400 cm.sup.3 iso-PrOH and left to stand to separate into two layers, the top of which contained 103 g of 97 percent hydrogenated polymer and the bottom of which contained 200 g of 56 percent hydrogenated polymer.
Japanese Patent 62151405 relates to OH-terminated diene polymers which are catalytically hydrogenated in high yield by solution hydrogenation, treatment with alcohols, and rehydrogenation of the resulting dark solution. Thus, 300 g of OH-terminated polyisoprene was hydrogenated in n-hexane with a Ni-kieselguhr catalyst and the solution treated with 400 g of 2-propanol to form transparent (hydrogenated to 97 percent) and dark-colored (hydrogenated to 56 percent) fractions. The colored fraction was rehydrogenated to obtain a hydrogenation ratio of 67 percent.
Japanese Patent 53026890 relates to using Raney Ni catalysts, to hydrogenate OH-containing unsaturated hydrocarbon polymers in alcohols or mixtures of alcohols and other organic solvents. Thus, 50 g of polybutadiene polyol (having a number average molecular weight of 3100 and an OH value of 0.82 mequiv./g) 50 g of iso-PrOH, and 5 g of Raney Ni catalyst were hydrogenated for 6 hours at 150.degree. and 50 kg/cm.sup.2 H pressure in an autoclave to give 50 parts polymer (having 0.83 mequiv./g OH value). The hydrogenation degree was 63.8 percent, as compared with 19.5 percent when cyclohexane was used instead of iso-PrOH.
Moreover, low molecular weight liquid conjugated diene-acrylonitrile copolymers are useful as additives in the processing of plastic and rubber polymers. It is well known to the art and to the literature that both the hydrocarbon and polar segments in such copolymers are essential for compatibility with the polymers being modified. In addition, the polar groups also aid in preventing additives, such as antioxidants, from migrating to the surface of the finished polymer product. However, such liquid copolymers have a major drawback associated with their use as additives, namely, their thermooxidative instability caused by unsaturation of the backbone and pendant vinyl groups resulting from the copolymeriztion of the conjugated diene. However, it has been found that the thermooxidative stability of such copolymers is significantly improved if the unsaturated copolymer is hydrogenated. Selective hydrogenation of the olefinic sites derived from the conjugated diene without affecting the unsaturation of the polar nitrile groups, results in a thermooxidatively stable polymer which retains the polar character of the unhydrogenated starting material. However, conventional low-cost homogeneous hydrogenation catalysts based on cobalt or nickel are unacceptable as hydrogenation catalysts for conjugated diene-acrylonitrile copolymers due to the tendency of the nitrile groups to undergo reduction, which results in loss of the polar character of the nitrile groups and hence the copolymer. Moreover, such reduction of the nitrile groups leads to undesirable crosslinking reactions during hydrogenation, yielding an unsuitable gelled product.
Heretofore, hydrogenation of such conjugated-acrylonitrile copolymers, and more particularly, selective hydrogenation of the olefinic sites derived from the conjugated diene without attendant unsaturation of the polar nitrile groups, has been achieved by the use of heterogeneous catalysts based on rhodium. Since such noble transition metal catalysts generally are about 1,000 times more expensive than cobalt or nickel, the high cost of the catalyst is reflected in the selling price of the end product, which is undesirable.
Unexpectedly, it has been found that the unsaturation derived from butadiene in butadiene-alkenylpyridine and butadiene-acrylate liquid copolymers can be selectively hydrogenated to a high degree using conventional homogenous catalysts based on cobalt or nickel. The choice of the pendant polar pyridyl and ester groups is due to the fact that these groups are much more difficult to hydrogenate than, say, the nitrile group in the butadiene-acrylonitrile copolymer discussed immediately above. These results are unexpected since addition of an unmodified homogeneous nickel or cobalt catalyst to the solution of a high molecular weight butadiene-alkenylpyridine or butadiene-acrylate copolymer causes gel formation, and it is well known to those having ordinary skill in the art that a gelled polymer cannot be hydrogenated to a high degree. Such gelation typically is due to crosslinking of the polymer by the catalyst via complexation with the polar pyridine nitrogen atoms and polar ester groups. In high molecular weight copolymers, as little as one or two crosslinks per 200 to 300 monomer units can cause gelation. However, in the low molecular weight liquid copolymers of the present invention, a much higher degree of chain extension and crosslinking must take place in order to cause gel formation. Thus, upon addition of an unmodified homogenous nickel or cobalt catalyst to the liquid copolymer, gelation unexpectedly does not occur. Hydrogenation then allows removal of the backbone and pendant unsaturation in the liquid copolymers of this invention without hydrogenation of the pendant polar pyridyl and ester groups. Thus, the thermooxidative stability of the liquid copolymers is improved, while the polar character of these materials is retained. Moreover, the retained polarity resulting from the pendant polar pyridyl and ester groups imparts compatibility to the copolymers for use as modifiers in other materials such as plastics and rubbers.
U.S. Pat. No. 3,416,899 (Schiff, Dec. 17, 1968) relates to improved gel compositions useful as incendiary fuels, as solid fuels for heating, as a fracturing liquid for subterranean formations, and the like. In another aspect, this reference relates to the preparation of hydrocarbon gel compositions by hydrogenating a hydrocarbon solution of an unsaturated rubbery polymer in the presence of a catalyst comprising a reducing metal compound and a salt of a Group VIII metal.
U.S. Pat. No. 3,673,281 (Bronstert et al., Jun. 27, 1972) relates to a process for the hydrogenation of polymers containing double bonds in solution and in the presence of a catalyst complex comprising:
A. a compound of iron, cobalt or nickel,
B. an organo-aluminum compound, and
C. hexaalkylphosphhoric acid triamide as activator.
Polymers of diene hydrocarbons contain double bonds in the backbone. These double bonds may be hydrogenated by conventional processes. Products which are wholly or partly hydrogenated in this way are superior to non-hydrogenated polymers in that they possess improved resistance to aging and are particularly resistant to oxidative degradation. In the case of block copolymers of dienes and vinyl aromatic compounds, in particular, the hydrogenated products also show improved tensile properties and mechanical strength. When only partially hydrogenated, the diene polymers may be vulcanized. Such vulcanizates possess a higher tensile strength and a lower glass temperature than vulcanizates of non-hydrogenated diene polymers.
U.S. Pat. No. 3,625,927 (Yoshimoto et al, Dec. 7, 1971) relates to a catalyst for hydrogenating a high molecular weight polymer having hydrogenatable unsaturated bonds. This catalyst is suitable for hydrogenation of the polymer is a viscous solution form and comprises a reaction product of (1) a metal chelate compound of nickel, cobalt, or iron, with (2) an organic metallic reducing agent in said chelate compound. The chelating agent is attached to the metal by a pair of nitrogen atoms and an oxygen atom.
U.S. Pat. No. 3,531,450 (Yoshimoto et al, Sep. 29, 1970) relates to a new hydrogenation catalyst consisting of three catalytic components and a process for hydrogenating polymers by the use of said catalyst. This three-component catalyst consists of (1) at least one kind of an unsaturated hydrocarbon selected from the group consisting of an olefinically unsaturated hydrocarbon and an acetylenically unsaturated hydrocarbon, (2) at least one kind of an organic compound of the metal selected from the group consisting of nickel, cobalt and iron, and (3) at least one kind of a metal compound reducing agent.
U.S. Pat. No. 3,766,300 (De La Mare, Oct. 16, 1973) discloses a process for the hydrogenation of copolymers prepared from conjugated dienes and certain copolymerizable polar monomers such as vinyl pyridines, acrylonitriles, and alpha-olefin oxides which comprises an initial step of forming a complex between at least one Lewis acid and the polar portions of the copolymer and thereafter subjecting the complex to hydrogenation. More particularly, this reference is especially concerned with a process for the hydrogenation of block copolymers derived from these monomers.
Japanese patent 13,615 (Aug. 2, 1967; filed Feb. 15, 1963) relates to copolymers of butadiene and vinyl pyridine that were reduced to give waterproof, stable reduced copolymers. These products were useful for coating pills. The reduced copolymers were obtained by the catalytic hydrogenation in the presence of Raney nickel catalyst.
A paper titled "Oil-Resistant Rubbers from 2-Methyl Vinyl Pyridine," James E. Pritchard and Milton H. Opheim, Industrial and Engineering Chemistry, Volume 46, No. 10, pages 2242-2245, relates to quaternization of liquid polymers. Copolymers of butadiene and 2-methyl-5-vinyl pyridine (MVP) react with quaternizing agents to form polymeric salts of the type: ##STR1## where R is an aliphatic or aromatic radical and X represents halide, alkyl sulfate, or aryl sulfonate groups.